arm: cmsis: Convert systick to CMSIS

Remove use of __scs structures and defines in place of CMSIS defined
ones.  Also, use __ISB() instead of inline asm.

Jira: ZEP-1568

Change-Id: I8798206a12680f6c50105c7c28112632ac9dde50
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>